1. Field of the Invention
The present invention is directed to electrical frequency measurement, and particularly to a signal processing apparatus for providing accurate temporal frequency measurement of radio frequency (xe2x80x9cRFxe2x80x9d) input signals over a wide instantaneous bandwidth and dynamic range.
2. Description of the Related Art
Electrical frequency measurement is a requirement for numerous signal processing, communication, and signal measurement systems in use today and in the foreseeable future. A wide instantaneous operating bandwidth is necessary in many such systems. Frequency is a key measurement parameter for the purpose of signal tracking, characterization, and identification in systems such as satellite and terrestrial communications systems, radar receivers, and RF test equipment. Frequency is one of the most useful parameters to differentiate between signals and to specifically identify a type of radar threat.
Present approaches to frequency measurement include frequency discriminator and instantaneous frequency measurement (xe2x80x9cIFMxe2x80x9d) devices for which the measured frequency accuracy is generally more sensitive to input signal strength variations. For example, most frequency discriminators operate within a limited internal input signal dynamic range, typically within the saturation region, and hence require additional circuits to constrain input signals to this region. For this reason these devices are unable to resolve pulse-on-pulse or pulse-on-continuous wave (xe2x80x9cCWxe2x80x9d) signal conditions well.
A U.S. patent of interest includes U.S. Pat. No. 5,682,238 to Levitt et al. (the subject matter of which is incorporated herein by reference), which discloses a signal processing apparatus with an optical phase measurement processor that provides phase difference measurements of multiple signal inputs.
The present invention relates to a signal processing apparatus and method that provide accurate temporal frequency measurement of RF input signals within a wide instantaneous input bandwidth and dynamic range. Instantaneous RF frequency measurement is accomplished using a hybrid optical/electronic processing architecture based on an optical frequency discriminator component and an associated spatial-domain phase measurement technique. An example of an efficient spatial sampling scheme for electrical phase measurement is described in the ""238 patent.
In the present invention, signal inputs of interest are individually detected, with concurrent frequency and possibly other time-domain parameter measurements, in sequence of occurrence. While optimized for sequential or xe2x80x9csingle-signal-at-a-timexe2x80x9d processing, the present invention also has the capability to process pulse-on-pulse inputs (e.g., pulse-modulated RF signals overlapped in time, but having different pulse start times) and pulse-on-CW background interference.
Basic frequency measurement occurs through two concurrent processes, (1) detection of signal presence with coarse frequency measurement, and (2) fine frequency measurement. Detection and coarse frequency measurement (i.e., a coarse indication of frequency subband) are accomplished using a conventional RF filter bank and detection circuit techniques. Fine frequency measurement incorporates a plurality of single channel frequency-to-phase translators, each having the form of an optical delay line frequency discriminator for which the resulting phase variation is encoded by an efficient optical spatial sampling technique. In a preferred embodiment, the translators are implemented in a suitable configuration, complemented with algorithms to extract phase measurements of individual translator elements and resolve circular phase ambiguities. The basic phase difference measurement element and decoding algorithm are realized as a single channel version of the apparatus described in the ""238 patent.
The multiple frequency discriminator outputs are applied to an ambiguity resolution algorithm to determine a unique (unambiguous) frequency measurement within a subband, and provide fine frequency resolution equivalent to the discriminator with maximum time delay. Frequency ambiguity resolution is accomplished with algorithmic methods similar to those employed by RF interferometers to resolve angle of arrival ambiguity. The encoded filter bank subband, in conjunction with fine frequency discriminator bank output, permits frequency measurement coverage of the entire system bandwidth.
Relative to commercial electronic frequency discriminator and instantaneous frequency measurement devices that incorporate fixed calibration techniques, the present invention provides improved and sustainable frequency measurement accuracy across the entire operating bandwidth and temperature range through continuous dynamic phase offset correction. Wide instantaneous RF bandwidth operation is achieved using developed optical modulation devices and RF filter bank technology to match or exceed available alternatives. Additionally, current frequency discriminator and IFM devices must function (internally) over a limited amplitude dynamic range to achieve acceptable measurement accuracy. The present invention functions correctly over a much wider dynamic range, limited only by noise and photodetector saturation. This feature may extend the achievable system-level dynamic range and/or simplify the system architecture by eliminating the requirement for saturated (or limited) mode operation, both results being advantageous in terms of system cost and complexity.
The present invention may be applicable to any receiver system that requires frequency measurement over a wide instantaneous bandwidth, such as electronic warfare (xe2x80x9cEWxe2x80x9d) receivers, electronic counter measures, radar warning receivers (xe2x80x9cRWRxe2x80x9d), shipboard electronic support measures (xe2x80x9cESMxe2x80x9d), unmanned aerial vehicles, and to direct broadcast satellite, cellular telephone, wireless LAN, and test and evaluation equipment. Frequency spectrum monitoring agencies may also find useful application of the present invention to enforce proper usage of the local RF spectrum.
These, together with other advantages that will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.